Critical Review Paper Provides Solutions for Eliminating Foam in Activated Sludge Plants

submitted by Denny Parker, Ph.D., P.E.*,
Director of Technology, Senior Vice President
Member, National Academy of Engineering
Brown and Caldwell | Walnut Creek, CA

WEF’s Water Environment Research (WER) recently included an article by Brown and Caldwell staff on their field research and designs for foam elimination in the activated sludge process. Since the beginning of the biological nutrient removal, many activated sludge processes have been plagued with nuisance foam problems. This occurs because the insertion of baffles causes foam trapping and the resulting enrichment of foam causing organisms into the mixed liquor. The organism’s surfaces are hydrophobic which gives them a propensity to attach to bubbles. When passed on to secondary clarification, this can cause floc to be floated to the surface of secondary clarifiers and be lost to the effluent. The entrained bubbles also lower settling velocities resulting in less compaction in secondary clarifiers, thereby reducing their capacity.

Figure 1. It takes two things to eliminate foam, such as been done in this plant. 1. Design baffles so foam is not trapped. 2. Place a Classifying Selector in service to select against foam causing organisms.

One of the original methods for control involved surface wasting of foam from aeration basins, which does not eliminate foam causing organisms but merely “manages” it.  Surface wasting transfers the foaming problem to subsequent processing steps. When foaming becomes severe, for example, the organisms remain hydrophobic even in anaerobic digesters, making digesters subject to flotation with attendant mechanical and safety issues.

The Classifying Selector, invented and proven in the field by Brown and Caldwell, deals with the problem by eliminating surface trapping in BNR plants and continuously wasting from the top few inches of mixed liquor in the aeration basins where the organisms are enriched. The wasting is continuous, which means that negative selection pressure keeps overall foam-causing organism counts at very low levels, so low that foam never forms. The method requires the use of no chemicals and is quite simple in operation, once the proper facilities have been put into service.

Formerly available to subscribers only, selected WER articles such as this one are available free to the public on a monthly basis through an open-access program. Click here to download the open access article, “A Critical Review of Nuisance Foam Formation and Biological Methods for Foam Management or Elimination in Nutrient Removal Facilities” by authors Denny Parker, John Bratby, Don Esping, Ted Hull, Rick Kelly, Henryk Melcer, Rion Merlo, Rod Pope, Todd Shafer, Eric Wahlberg, and Robert Witzgall.

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